The present invention relates generally to a method of installing a stent utilizing a balloon catheter to perform an initial angioplasty and to seat the stent after it has been located in the vessel. The invention also relates to novel balloon structures which have particular use in the method of the invention.
Angioplasty, an accepted and well known medical practice involves inserting a balloon catheter into the blood vessel of a patient, maneuvering and steering the catheter through the patient""s vessels to the site of the lesion with the balloon in an uninflated form. The uninflated balloon portion of the catheter is located within the blood vessel such that it crosses the lesion or reduced area. Pressurized inflation fluid is metered to the inflatable balloon through a lumen formed in the catheter to thus dilate the restricted area. The inflation fluid is generally a liquid and is applied at relatively high pressures, usually in the area of six to twenty atmospheres. As the balloon is inflated it expands and forces open the previously closed area of the blood vessel. Balloons used in angioplasty procedures such as this are generally fabricated by molding and have predetermined design dimensions such as length, wall thickness and nominal diameter. Balloon catheters are also used in other systems of the body for example the prostate and the urethra. Balloon catheters come in a large range of sizes and must be suitably dimensioned for their intended use.
Recently the use of a catheter delivered stent to prevent an opened lesion from reclosing or to reinforce a weakened vessel segment, such as an aneurism, has become a common procedure. A typical procedure for stent installation involves performing an initial angioplasty to open the vessel to a predetermined diameter sufficent to permit passage of a stent delivery catheter across the lesion, removal of the angioplasty balloon catheter, insertion of a delivery catheter carrying the stent and a stent deploying mechanism, deploying the stent across the opened lesion so as to seperate the stent from the catheter and bring it into contact with the vessel wall, usually with dilation to a larger diameter using a balloon larger than the balloon of the predilation catheter, and then removing the delivery catheter (after deflating the balloon if used). In many cases it has become the practice to then xe2x80x9cretouchxe2x80x9d the dilation by deploying a third catheter carrying a balloon capable of dilating at a substantially higher pressure to drive the stent into the vessel wall, thereby to assure that there is no risk of the stent later shifting its position and to reduce occurance of restenosis or thrombus formation. This third xe2x80x9cretouchxe2x80x9d dilation is often considered necessary when the balloon used to seat the stent is made of a compliant material because such balloons generally cannot be safely pressurized above 9-12 atm., and higher pressures are generally considered necessary to assure full uniform lesion dilation and seating of the stent.
A wide variety of stent configurations and deployment methods are known. For instance, stent configurations include various forms of bent wire devices, self-expanding stents; stents which unroll from a wrapped configuration on the catheter; and stents which are made of a deformable material so that the device may be deformed on deployment from a small diameter to a larger diameter configuration. References disclosing stent devices and deployment catheters include:
In U.S. Pat. No. 5,348,538, incorporated herein by reference, there is described a single layer balloon which follows a stepped compliance curve. The stepped compliance curves of these balloons has a lower pressure segment following a first generally linear profile, a transition region, typically in the 8-14 atm range, during which the balloon rapidly expands yielding inelastically, and a higher pressure region in which the balloon expands along a generally linear, low compliance curve. The stepped compliance curve allows a physician to dilate different sized lesions without using multiple balloon catheters.
Stepped compliance curve catheter balloon devices using two different coextensively mounted balloon portions of different initial inflated diameter, are also described in co-pending U.S. application Ser. No. 08/243,473, filed May 16, 1994 as a continuation of now abandoned U.S. application Ser. No. 07/927,062, filed Aug. 8, 1992, and in U.S. Pat. No. 5,358,487 to Miller. These dual layer balloons are designed with the outer balloon portion larger than the inner portion so that the compliance curve follows the inner balloon portion until it reaches burst diameter and then, after the inner balloon bursts, the outer balloon becomes inflated and can be expanded to a larger diameter than the burst diameter of the inner balloon.
A polyethylene ionomer balloon with a stepped compliance curve is disclosed in EP 540 858. The reference suggests that the balloon can be used on stent delivery catheters. The disclosed balloon material of this reference, however, yields a compliant balloon and therefore a stent delivered with such a balloon would typically require xe2x80x9cretouch.xe2x80x9d
The invention in one aspect is directed to a method for installing a stent in a vessel utilizes a single balloon catheter for both low pressure predilation at a relatively small diameter to open the lesion sufficiently to allow insertion and deployment of the stent across the lesion and for subsequent high pressure embedding of the stent in the vessel wall. The same balloon catheter may also be employed to insert and deploy the stent. Thus at least one catheter may be eliminated from what has heretofore been a two or three catheter installation process. The balloons utilized in the method have a stepped compliance curve which allows for predilation at a low pressure and predetermined diameter and for high pressure embedding at a substantially larger diameter.
In a further aspect of the invention novel balloon structures having high wall strengths, high burst pressures and low compliance are provided in which a first portion of the balloon body has a generally linear compliance curve and a second portion of the balloon body has a stepped compliance curve. Both portions of the balloon are configured to have essentially the same diameter at low pressure so that the entire balloon may be used to predilate a lesion. However at higher pressure the configuration of the balloon changes due to rapid expansion of the second balloon portion. At still higher pressures the compliance curve of the second portion levels off to a low compliance profile so that this portion of the balloon can be used for high pressure embedment of the stent without substantially increasing the stent size. With such balloons, exposure of the vessel wall areas which are not reinforced by the stent to high pressure can be avoided, despite the typically shorter length of conventional stents than the typical length of predilation balloons.
The novel balloons of the invention are made by molding a balloon into a configuration in which the second portion has a larger diameter than the first portion and then shrinking the second portion to the diameter of the first portion. The method of making such balloons comprises yet another aspect of the invention.
These and other aspects and advantages of the present invention will no doubt become apparent to those skilled in the art after having read the following detailed description of the invention as illustrated by the various drawing figures.